In the reciprocating hermetic compressors operated by a piston and normally used in small refrigeration systems or appliances, the compression of the refrigerant gas is obtained by the reciprocating movement of the piston inside a cylinder, between the displacement limits determined by the driving mechanism known as the lower dead point and the higher dead point. The cylinder has an open end, and an opposite end closed by a valve plate that defines one of the ends of a compression chamber, which other end is defined by the piston top.
In order that the movement of the piston inside the cylinder occurs in an adequate manner, it is necessary to provide a radial gap between the piston of smaller diameter and the cylinder of larger diameter.
During the operation of the compressor, said radial gap is partially filled with lubricant oil, bearing the piston and preventing wear between the movable parts. This bearing leads to a dissipation of mechanical energy, in order to overcome the viscous friction produced by the oil and by the movement of the piston.
When the piston is displaced from the lower dead point to the higher dead point, the refrigerant gas is compressed in the compression chamber, increasing its pressure in relation to the pressure of the gas existing inside the shell of the compressor, and causing a pressure differential that tends to produce leakage inside the shell, through the radial gap, of part of the refrigerant gas being compressed in the compression chamber. This phenomenon characterizes a volumetric loss and reduces the refrigeration capacity of the compressor, since the leakage causes a compression operation to be performed on a certain quantity of refrigerant gas that is directed to the inside of the shell. This loss directly reduces the energetic efficiency of the compressor.
The bearing of the piston, as well as the leakage of the compressed gas are functions mainly of the diameters and lengths of the cylinder and piston, of the distance traveled by the piston, of the rotational speed of the driving shaft, of the geometry of the driving mechanism, of the type of refrigerant gas used, of the type of lubricant oil, and of the operational conditions of the compressor (pressures and temperatures).
The piston is connected to the driving mechanism, which generally comprises a connecting rod, by means of a pin having opposite ends lodged and retained in a pair of diametrically opposite radial holes provided in the median region of the piston. The assembly of the pin to the radial holes of the piston is made, so that these two opposite regions represent a restriction to the flow of refrigerant gas that leaks by the radial gap existing between the cylinder and a piston top bearing surface, which is defined between the piston top and a transversal plane containing the pin axis, said restriction being smaller than the restriction to said flow of refrigerant gas imposed by the radial gap existing between the cylinder and the piston bottom bearing surface defined between the bottom of the piston and said transversal plane. Thus, in the known assemblies, the refrigerant gas tends to leak by the radial gap from the top of the piston to the inside thereof, through median radial holes of the piston. Thus, only the piston top bearing surface has also the function of restricting the leakage of gas through the radial gap, mainly during the compression cycle, since in the suction cycle the reverse leakage that may occur can be ignored, besides being considered positive in terms of volumetric efficiency of the compressor. However, the piston bottom bearing surface, which is defined between the bottom of the piston and the pin, and which does not have a restrictive function against the leakage of refrigerant gas, leads to power dissipation by viscous friction.
The high efficiency compressors have an external circumferential recess in the median region of the piston, in which the radial holes are provided for mounting the pin, separating the above mentioned top and bottom bearing surfaces. This artifice is used to reduce the power dissipated in the bearing of the piston, without increasing the leakage of refrigerant gas by the radial gap, since the axial extension of the piston top bearing surface is maintained at a minimum value that is required to assure a determined restriction to the flow of refrigerant gas through the radial gap in this region. The piston bottom bearing surface is maintained to guide the piston, producing viscous friction and having no positive effect considered relevant to restrict the leakage of refrigerant gas.